Thick film resistors, including thermistors, are ordinarily made by sintering a mixture of suitable semiconducting oxides and a vitreous binder onto a desired substrate. The semiconductive oxides provide a desired compound which provides a suitable chemical and/or physical structure that produces the predominant electrical characteristics of the film. The vitreous binder, i.e. a glass, enhances mechanical properties of the film and bonds it to an underlying substrate. It can also serve as an aid in sintering. In essence, a sintering aid is needed to obtain faster sintering at lower temperatures. Ordinarily only minor proportions of the sintering aid are needed to enhance sintering. Major proportions of it can deleteriously affect the film, as for example significantly raise electrical resistivity. However, sufficient vitreous binder must be present to provide resistance to both abrasion and environmental degradation. On the other hand, too much vitreous binder can cause resistivity of the film to be too high. For the foregoing reasons, it has been extremely difficult to form low resistance thermistor cermet thick films. In some instances, for example, for thermistors where very low resistance and high change in resistance with temperature is desired, it is not possible to obtain such a characteristic unless the glass content is kept so low that the film is no longer durable.
We have now found that a thermistor composition and method of making it that permits us to obtain both low resistivity and high durability. In addition, we have found a particular thermistor composition that provides these characteristics when normally co-fired with ordinary resistors and conductors.